Batteries having the combined characteristics of high capacity, high power, rechargeability, long discharge/charge cycle life, minimum size and weight, economy of manufacture, and environmental safety remain to be developed.
Zinc (Zn) has been used in many battery applications as the anode material. This is due to the high energy capacity of zinc and to its chemical stability in the electrolyte. Zinc electrodes provide high current densities and a flat discharge curve. Battery systems such as nickel-zinc, silver-zinc, zinc-chloride, zinc-bromide, zinc-manganese etc. are well known. A zinc electrode can be made from a solid plate, pellets or powdered zinc materials.
If a powdered zinc material is used for the electrode, an organic gelling agent may be added to allow sufficient electrolyte penetration and to maintain Zn—Zn particle contact area. This is important in order to obtain high electronic conductivity. However, there are problems associated with the use of such gelling agents, including their tendency to break up due to repeated swelling and contraction of the zinc electrode, destruction of the uniformity of distribution of zinc powder by impact and the formation of passive layers under high discharge current densities. Accordingly an object of the present invention is to provide an electrode with improved Zn—Zn particle contact area.
Reaction rate at a zinc electrode made from zinc powder is dependent on active surface area and so theoretically shape and available surface area of the zinc particles should be optimal to give best performance. Various additives have also been used in zinc electrodes in an attempt to increase activity and stability. For example, graphite has been added to electrodes to increase particle-particle contact area and calcium zincate has been used to improve the stability.
A further object of the present invention is to provide a zinc electrode with increased activity and stability.
For secondary batteries with zinc as the active anode material, low cost and relatively high energy density can be obtained but this is offset by the short cycle life of the battery. Charging the zinc electrode results in deposition of zinc on the electrode surface and on previously deposited zinc in the form of zinc crystals or dendrites. The build up of zinc on the outer electrode surface may block the interior electrode surface and reduce the capacity of the system after repeated charge and discharge cycling. In addition zinc dendrites spread out forming needle like zinc crystals penetrating the cell separator and causing short circuit of the battery system.
Shape change is defined as the migration of zinc. Most common is the migration from the top and sides of the electrode to the bottom. This is due in part to the dissolution of zinc during the discharge cycle. Zinc, in oxide form, and partially dissolved in potassium hydroxide, is moved to other areas of the electrode. Movement occurs down the concentration gradient of the dissolved zinc hydroxides and so zinc is moved to other areas of the electrode, to a great extent in the lower portion, where the zinc is redeposited on charge. During charge/discharge cycles, the general tendency is for zinc to leave certain areas of the electrode and concentrate in other areas. Usually zinc is concentrated in the lower part of the electrode due to gravitational forces, causing the lower portion of the electrode to become denser than the upper part. Potential differences over the electrode surface may also result in local shape changes.
Prevention of this zincate diffusion within the battery is the main challenge for obtaining rechargeable zinc batteries. Zinc is extremely soluble in strong alkaline environments. The high solubility allows for rapid current spikes typically unattainable with other battery systems. However, zinc diffusion leads to the well-known phenomena of electrode shape change and the presence of zinc dendrites within the battery.
Attempts to control shape change and dendrite formation include:                redistribution of zinc by starting out with zinc depleted at the centre and agglomerated at the edge;        modifying the electric field experienced by the zinc electrode;        using separators resistant to zinc dendrites;        decreasing the solubility of zinc by using complexing agents.        
Incorporation of binders such as polytetrafluoroethylene (PTFE) in battery electrodes in an attempt to reduce shape changes is also known, but one particular problem in this context is that when PTFE alone is incorporated into the electrode, the PTFE tends to coagulate as a film on the outside of the electrode during cycling. This not only reduces the useful effect of the binder in avoiding shape changes but also increases the resistance of the electrochemical cell, including the electrode.
French Patent No. 2264401 describes a secondary zinc electrode which is produced by applying to a collecting grid a non-hardened mixture containing particulate zinc oxide, a binder such as PTFE, and other substances, such as rayon fibres or metal powders. However, this electrode still suffers from problems of dendritic growth and, in particular, shape changes or deformations, probably as a result of movement of the electrolyte parallel to the surface of the electrode during the charge and discharge cycles.
A further possible approach to controlling shape change and dendrite formation in rechargeable zinc electrodes is by containing the zinc in a matrix by encapsulation, typically using a gelling agent. Organic gelling agents with high ionic conductivity, such as acrylate polymers (e.g. CARBOPOL), can be used to encapsulate zinc powder. With the use of gelling agents the multiplicity of pores and the high tortuous paths within the resulting matrix limit the mobility of zinc oxide/zinc hydroxide during the discharge of a battery and, thereby, retard the migration of zinc, which is equivalent to shape changes in a conventional zinc electrode. Thus the cycling of the battery is extended.
A problem with such gelling agents is their tendency to break up due to repeated swelling and contraction of the zinc electrode which occurs due to the volumetric difference between the metallic zinc and the zinc oxides because the density of ZnO is lower than that of Zn. As the electrolyte is saturated with zincate, ZnO (zinc oxide) will be deposited. With encapsulated zinc particles this deposition will occur inside the encapsulation and as a consequence changes in the size of the electrode occur which can break up the encapsulating effect of the gelling agent around the zinc particles.
Due to the density variations in the electrode the encapsulating effect of the polymer gelling agent is removed. Open channels are produced in the electrode structure allowing transport of zinc ions and subsequently the build-up of dendrites or shape changes on the electrode during charging.
A further problem with such gelling agents is the swelling nature of the agents when in contact with the alkaline electrolyte (e.g. KOH) due to incorporation of the electrolyte into the organic polymer. Such swelling of the polymer may result in both positive and negative effects on the electrode. Severe swelling results in an increased particle contact resistance within the electrode as the zinc particles are removed from each other under repeated charge/discharge cycling. This reduces the capacity of the system and limits the number of charge/discharge cycles. The current collector pressed into the electrode or implemented in the battery can also suffer from these effects as part of it loses electronic contact with the zinc matrix. In addition dry spots can occur in the electrode, reducing the utilisation of zinc.
However, if the electrode swells to a lesser extent a positive effect may be observed in that it may result in the formation of a thin electrolyte layer around the zinc particles which can give superior discharge characteristics.
Further drawbacks with such gelling agents are destruction of the uniformity of distribution of zinc powder by impact and the formation of passive layers under high discharge current densities.
An object of the present invention is to provide a zinc electrode which is stable after repeated charge/discharge cycling.